This invention generally pertains to methods and apparatus for heat treatment. More particularly, the invention relates to batch coil annealing furnaces and a valving system therefor and will be described with particular reference thereto. However, it will be appreciated by those skilled in the art that the invention has broader applications and may generally be applied to valving systems for all types of heat treating operations where at least one work item is placed within an enclosure in a heat transfer relationship with a heating means within the enclosure.
Metals are annealed to reduce their hardness, and improve machinability, facilitate cold working, produce a desired microstructure or to obtain desired mechanical, physical or other properties. When applied to metals, the term "annealing" without qualification implies full annealing which is defined as heating a metal or alloy to its austenitizing temperature and then cooling it slowly through the transformation range under controlled conditions according to a predetermined schedule. There are also many types of partial annealing, such as black annealing, blue annealing, box annealing, bright annealing and so forth.
Annealing of metal strips and the like is generally accomplished by winding the strips into coils having an axial passage bounded by the inner diameter of the winding. Several coils can be stacked on top of one another and enclosed in a sealed inner cover. The inner cover defines a work space and is, itself, enclosed in an outer furnace chamber. Heat is transferred from the outer furnace chamber to the inner cover which, in turn, transfers the heat to the coils. The primary mode of heat transfer from the cover to the coils is by radiation. Additionally, a gas atmosphere is circulated within the inner cover to achieve a more rapid and uniform heat transfer by convection. For proper annealing, a suitable gas atmosphere for annealing has to be maintained within the work space.
The effect of a heat treating operation on the surface condition of workpieces is influenced by the time of heating, the temperature level maintained, and by the atmosphere surrounding the material. By using the proper atmosphere in the working chamber, a clean scale-free surface can be obtained. Such a surface is required for most sheet and strip material and other important products, such as steel wire and tubes. However, some gases, such as oxygen, carbon dioxide and water vapor are very injurious to steel products whereas other gases, such as nitrogen and hydrogen are neutral or even beneficial.
Therefore it is common to use nitrogen, hydrogen or mixtures thereof as a treatment atmosphere during annealing. For example, one common gas mixture which is used for annealing work is composed of 95% nitrogen and 5% hydrogen. Alternatively, a pure nitrogen atmosphere or another nitrogen-hydrogen mixture may be used. Generally, low percentages of hydrogen are used both because hydrogen is quite flammable and because hydrogen is considerably more expensive than nitrogen. Of course, other types of gases, such as methane and the like, can also be used in the treatment atmosphere provided during the annealing process.
One problem with such annealing gases, and particularly nitrogen and hydrogen, is that they are quite expensive to use. During the heating phase of the annealing process, however, the treatment gas atmosphere has to be continuously flushed through the work space. In one conventional process of annealing, a continuous flushing with new gas is also required during the cooling phase since if the gas outlet valve is closed, gas pressure inside the inner cover may become too high due to a pressure build-up caused by a heating of the treatment atmosphere gas by the hot material which is being annealed. If such pressurized gas is not exhausted after being so heated, the inner cover of the furnace would be lifted off its base and begin to float thus mixing the neutral gas atmosphere within the work space with deleterious gases such as oxygen and carbon dioxide.
Accordingly, it has been considered desirable to develop a new and improved coil annealing furnace which would overcome the forgoing difficulties and others while providing better and more advantageous overall results.